Physical random access channel for slotted wireless communication systems

ABSTRACT

In one embodiment, a physical random access channel (PRACH) is used in communicating in a slotted wireless communication system. The slotted system uses repeating radio frame structure for communication. The PRACH is associated with a set of characteristics comprising: a set of channelization codes associated with the PRACH; at least one time slot associated with the PRACH; and a set of subchannels associated with the PRACH, the subchannels associated with a single time slot of the radio frames. In another embodiment, access service classes (ASCs) are separted in a wireless slotted communication system. Each ASC is associated with a unique set of channelization codes and subchannels of a physical random access channel (PRACH). The subchannels are associated with a single time slot of radio frames of the wireless slotted communication system.

CROSS REFERENCE TO RELATED APPLICATION(S)

[0001] This application is a continuation of U.S. patent applicationSer. No. 10/020,725, filed Dec. 12, 2001, which claims priority fromU.S. Provisional Patent Application No. 60/256,621, filed on Dec.19,2000, which are incorporated by reference as if fully set forth.

BACKGROUND

[0002] The invention generally relates to wireless time division duplex(TDD) communication systems using code division multiple access. Inparticular, the invention relates to sub-channels for the physicalrandom access channel (PRACH) for such systems.

[0003] In code division multiple access (CDMA) communication systemsusing frequency division duplex (FDD), such as proposed for the thirdgeneration partnership project (3GPP), physical random access channels(PRACHs) are used for transmitting infrequent data packets and systemcontrol information from the user equipments (UEs) or users to theNode-B.

[0004] In a 3GPP FDD/CDMA system, the PRACH is divided into ten (10)millisecond radio frames 22 ₁ to 22 ₈ (22) having fifteen (15) timeslots24, as shown in FIG. 1. The radio frames 22 are sequentially numbered,such as numbered from 0 to 255, as a system frame number. The systemframe numbers are sequentially repeated. The random access transmissionstarts at the beginning of a number of well-defined time intervals,denoted access slots 26. The random access transmissions 28 ₁ to 28 ₅(28) from the users are begun in a particular access slot 26 andcontinue for one or multiple slots 26. These transmissions are sentusing a randomly selected signature associated with an access serviceclass (ASC) assigned by a radio resource controller of the network tothe user.

[0005] The PRACH is used for infrequent data packets and system controlinformation and the network uses sub-channels of the PRACH for furtherseparation of UEs and access service classes. In the 3GPP FDD/CDMAsystem, each sub-channel is associated with a subset of the total uplinkaccess slots 26, described as follows.

[0006] Two sequential radio frames 22 are combined into one access frame20. The access frame is divided into 15 access slots 26. Each accessslot 26 has a duration of two radio frame timeslots 24 as shown inFIG. 1. The duration of a radio frame 22 is shown in FIG. 1 by the dualheaded arrows. The sub-channels are assigned to the access slots 26 bysequentially numbering the slots from 0 to 11, as shown in FIG. 1. Aftersub-channel 11 is assigned, the next access slot 26 is numbered 0 andthe numbering is repeated. The access slot 26 to sub-channel numberingis repeated every 8 radio frames or 80 milliseconds (ms). Thisrepetition can be viewed as a modulo (mod) 8 counting of the radio framenumbers.

[0007] In 3GPP FDD/CDMA, multiple PRACHs are used. Each PRACH isuniquely associated with a random access channel (RACH) transportchannel and is also associated with a unique combination of preamblescrambling code, available preamble signatures and availablesub-channels.

[0008]FIG. 2 is one example of an illustration of such an association.RACH 0 30 ₀ is paired with PRACH 0 32 ₀ through a coding block 31 ₀. Thedata received over PRACH 0 32 ₀ is recovered using the preamblescrambling code 0 34 ₀ and the appropriate preamble signature 38 thatthe data was sent.

[0009] PRACH 0 32 ₀ is uniquely associated with preamble scrambling code0 34 ₀ and has three access service classes (ASCs), ASC0 40 ₀, ASC1 40 ₁and ASC2 40 ₂. Although the number of ASCs shown in this example arethree, the maximum number of ASCs is eight (8). Each ASC 40 has a numberof available sub-channels, available preamble signatures and apersistence factor. The persistence factor represents the persistence inretransmitting the preamble signature after a failed access attempt. In3GPP FDD/CDMA, the maximum available sub-channels 36 is 12 and themaximum available preamble signatures 38 is 16.

[0010] RACH 1 30 ₁ is paired with PRACH 1 32 ₁. PRACH 1 32 ₁ is uniquelyassociated with preamble scrambling code 1 34 ₁ and its sub-channels 36and preamble signatures 38 are partitioned into four ASCs 40, ASC0 40 ₃,ASC1 40 ₄, ASC2 40 ₅ and ASC3 40 ₆. RACH 2 30 ₂ is paired with PRACH 232 ₂. PRACH 2 32 ₂ uses preamble scrambling code 2 34 ₂, which is alsoused by PRACH 3 32 ₃. Three ASCs 40 are available for PRACH 2 32 ₂, ASC040 ₇, ASC1 40 ₈ and ASC2 40 ₉. Because PRACH 2 and PRACH 3 share thepreamble scrambling code, a group of partitioned off availablesub-channels/available preamble signature combinations are not used forPRACH 2 32 ₂. The partitioned off area is used by PRACH 3 32 ₃.

[0011] RACH 3 30 ₃ is paired with PRACH 3 32 ₃. PRACH 3 32 ₃ also usespreamble scrambling code 2 34 ₂ and uses ASC0 40 ₁₀ and ASC1 40 ₁₁. ASC040 ₁₀ and ASC1 40 ₁₁ contain the available sub-channel/signature set notused by PRACH 2 32 ₃.

[0012] Since each PRACH ASC 40 is uniquely associated with a preamblescrambling code 34 and available preamble signatures set andsub-channels, the Node-B can determine which PRACH 32 and ASC 40 isassociated with received PRACH data. As a result, the received PRACHdata is sent to the appropriate RACH transport channel. Although eachPRACH 32 is illustrated in this example by having the ASCs 40partitioned by available preamble signatures, the partitions may also beby sub-channel 36.

[0013] Another communication system proposed to use PRACHs is a CDMAsystem using time division duplex (TDD), such as the proposed 3GPPTDD/CDMA system. In TDD, radio frames are divided into timeslots usedfor transferring user data. Each timeslot is used to transfer onlyuplink or downlink data. By contrast, an FDD/CDMA system divides theuplink and downlink by frequency spectrum. Although the air interface,physical layer, between FDD and TDD systems are quite different, it isdesirable to have similarities between the two systems to reduce thecomplexity at the network layers, such as layer 2 and 3.

[0014] Accordingly, it is desirable to have sub-channels for the RACHfor TDD.

SUMMARY

[0015] In one embodiment, a physical random access channel (PRACH) isused in communicating in a slotted wireless communication system. Theslotted system uses repeating radio frame structure for communication.The PRACH is associated with a set of characteristics comprising: a setof channelization codes associated with the PRACH; at least one timeslot associated with the PRACH; and a set of subchannels associated withthe PRACH, the subchannels associated with a single time slot of theradio frames.

[0016] In another embodiment, access service classes (ASCs) are separtedin a wireless slotted communication system. Each ASC is associated witha unique set of channelization codes and subchannels of a physicalrandom access channel (PRACH). The subchannels are associated with asingle time slot of radio frames of the wireless slotted communicationsystem.

BRIEF DESCRIPTION OF THE DRAWING(S)

[0017]FIG. 1 is an illustration of access slots and sub-channels for aFDD/CDMA system.

[0018]FIG. 2 is an illustration of PRACH configurations in a FDD/CDMAsystem.

[0019]FIG. 3 is an illustration of sub-channels in a time divisionduplex (TDD)/CDMA system.

[0020]FIG. 4 is an illustration of PRACH configurations in a TDD/CDMAsystem.

[0021]FIG. 5 is a simplified diagram of a Node-B/base station and a userequipment using a TDD/CDMA PRACH.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0022] Although the following discussion uses a 3GPP system forillustration, sub-channels for a TDD PRACH is applicable to othersystems.

[0023]FIG. 3 illustrates a preferred implementation of sub-channels fortimeslot 3 for PRACHs of a TDD/CDMA system. Each PRACH 48 is associatedwith one timeslot number 56 and a set of sub-channels 50 andchannelization codes 52, as shown in FIG. 4. For a particular timeslotnumber 56, a sub-channel 50 is uniquely associated with a radio frame44, as shown by double ended arrows. In a preferred implementation, suchas shown in FIG. 3, each sub-channel 50 is sequentially assigned tosequential radio frames 44. To illustrate, sub-channel 0 is associatedwith a timeslot number of a jth radio frame, such as radio frame 0 ofFIG. 4. Sub-channel 1 is associated with the same timeslot number of thenext (j+1^(th)) radio frame, such as radio frame 1.

[0024] After n radio frames, the next n frames are assigned the samesub-channels 50. For instance, sub-channel 0 is assigned to radio framen+j, such as radio frame n. For a particular timeslot 56, thesub-channels 50 are assigned based on the system frame number, which isa series of repeating radio frames. A preferred scheme uses a modulofunction of the system frame number (SFN) for n sub-channels. Forsub-channel i, Equation 1 is used.

SFN mod n=i   Equation 1

[0025] mod n is a modulo n function. One illustration uses a modulo 8function, such as per Equation 2.

SFN mod8=i   Equation 2

[0026] As a result, as shown in FIG. 3, in a first frame 44 ₀ intimeslot 3, sub-channel 0 is assigned. In a second frame 44 ₁,sub-channel 1 is assigned and so on until an eighth frame 44 ₇ wheresub-channel 7 is assigned. Preferably, the number of sub-channels is8,4, 2 or 1. Although FIG. 3 only illustrates sub-channel assignmentsfor timeslot 3, the same scheme is used on any timeslot number.

[0027] In a FDD/CDMA system, each PRACH 32 is associated with a uniquecombination of preamble scrambling code 34, available sub-channels 36and available preamble signatures 38. One example of a potentialimplementation of 4 PRACHs is shown in FIG. 4.

[0028] In an analogous manner, each PRACH 48 in a TDD system ispreferably associated with a unique combination of timeslot 56,available channelization codes 50 (preferred a maximum of 8) andavailable sub-channels 52 (preferred maximum of 8) as shown in FIG. 4.The channelization codes 52 are used by the users to transmit the uplinkdata. Similar to FDD, each TDD PRACH 48 is paired with a RACH 46transport channel via a coding block 47. FIG. 4 illustrates a generalconfiguration for the PRACHs 48. Each PRACH 48 is associated with atimeslot 56 and a set of available sub-channels 50 and availablechannelization codes 52. As shown in FIG. 4, each PRACH 48 in aparticular timeslot is assigned exclusive channelization codes 52. Thisallows the base station PRACH receiver to distinguish between thedifferent PRACHs 48 by knowing the channelization codes 52 used torecover the received PRACH data.

[0029] ASCs 54 are preferably formed by partitioning a particularPRACH's available sub-channels 50 and channelization codes 52.Typically, a limit is set for the number of ASCs 54, such as eight (8).RACH 0 460 receives data over PRACH 0 480 by decoding data transmittedin timeslot 0 560 with the appropriate channelization codes of PRACH 0480. The available sub-channels 50 and channelization codes 52 arepartitioned into three ASCs 54, ASC0 540, ASC1 541 and ASC2 542. Asshown, each partition is set by channelization codes 52, although, inanother implementation, the partitions may be by sub-channels 36 or aunique set of channelization code/sub-channel combinations. As a resultin the present example, each ASC 54 has a unique set of channelizationcodes 52 for that PRACH 48. The ASC 54 associated with received PRACHdata is determined using the channelization code 52 used to recover thereceived PRACH data.

[0030] RACH 1 46 ₁ receives data over PRACH 1 48 ₁ by decoding datatransmitted in timeslot 1 56 ₁ using PRACH 1's channelization codes 52.The available sub-channels 50 and channelization codes 52 arepartitioned into four ASCs 54, ASC0 54 ₃, ASC1 54 ₄, ASC2 54 ₅ and ASC354 ₆.

[0031] RACH 2 46 ₂ receives data over PRACH 2 48 ₂ by decoding datatransmitted in timeslot 2 56 ₂ using PRACH 2's channelization codes 52.The available sub-channels 50 and channelization codes 52 arepartitioned into three ASCs 54, ASC0 54 ₇, ASC1 54 ₈ and ASC2 54 ₉, andan unavailable partition used for PRACH 3 48 ₃. RACH 3 46 ₃ receivesdata over PRACH 3 48 ₃ by decoding data transmitted in timeslot 2 56 ₂using PRACH 3's channelization codes 52. The available sub-channels 50and channelization codes 52 for timeslot 2 56 ₂ are partitioned into twoASCs 54, ASCO 54 ₁₀ and ASC1 54 ₁₁ and an unavailable partition used byPRACH 2 48 ₂. As shown in FIG. 4, timeslot 2 56 ₂ is effectively dividedinto two PRACHs 48, PRACH 2 48 ₂ and 3 48 ₃, by channelization codes 52.As a result in this example, data received in timeslot 2 56 ₂ is sent tothe appropriate PRACH 48 based on the channelization codes used totransmit the data. Alternately in another implementation, the partitionmay be by sub-channels 36 or channelization code/sub-channelcombinations.

[0032] As shown in the PRACH implementation of FIG. 4, the example ofthe TDD PRACH configuration is analogous to the example FDD PRACHconfiguration of FIG. 2. In TDD, each PRACH is associated with atimeslot 56. In FDD, each PRACH is associated with a preamble scramblingcode 34. TDD ASCs 54 are preferably partitioned by availablechannelization codes 52 and FDD ASCs 40 by available preamble signatures38. These similarities for these examples allow for the higher layers tooperate similarly between TDD and FDD.

[0033]FIG. 5 is a simplified block diagram of a TDD PRACH system. Foruse in sending PRACH information, such as an assigned PRACH and ASC, tothe UE 60 from the network controller 62 via the Node-B/base station 58,a PRACH information signaling device 66 is used. The PRACH informationsignal passes through a switch 70 or isolator and is radiated by anantenna 72 or an antenna array through a wireless radio channel 74. Theradiated signal is received by an antenna 76 at the UE 60. The receivedsignal is passed through a switch 78 or isolator to a PRACH informationreceiver 82.

[0034] To send data over the PRACH from the UE 60 to the base station58, a PRACH transmitter 80 spreads the PRACH data 84 with one of theavailable codes for the PRACH assigned to the UE 60 and time multiplexesthe spread data with the timeslot of that PRACH. The spread data ispassed through a switch 78 or isolator and radiated by an antenna 76through a wireless radio interface 74. An antenna 72 or antenna array atthe base station 58 receives the radiated signal. The received signal ispassed through a switch 70 or isolator to a PRACH receiver 68. The PRACHdata 84 is recovered by the PRACH receiver 68 using the code used tospread the PRACH data 84. The recovered PRACH data 84 is sent to theRACH transport channel 64 ₁-64 _(N) associated with that PRACH. Thenetwork controller 62 provides PRACH information to the PRACH receiver68 for use in recovering the PRACH data 84.

What is claimed is:
 1. A physical random access channel (PRACH) for usein communicating in a slotted wireless communication system, the slottedsystem using repeating radio frame structure for communication, thePRACH associated with a set of characteristics comprising: a set ofchannelization codes associated with the PRACH; at least one time slotassociated with the PRACH; and a set of subchannels associated with thePRACH, the subchannels associated with a single time slot of the radioframes.
 2. The PRACH of claim 1 wherein the radio frames have associatedsystem frame numbers (SFNs) and each sub channel is associated withspecified ones of the SFNs.
 3. The PRACH of claim 1 wherein theassociating each subchannel with specified ones of the SFNs is by amodulo counting of the SFNs.
 3. The PRACH of claim 1 wherein a number ofthe subchannels is N and N has a value of either 1, 2, 4 or
 8. 4. ThePRACH of claim 3 wherein an ith subchannel is associated with a SFN byi=SFN mod N.
 5. A method for separating access service classes (ASCs) ina wireless slotted communication system, the method comprising:assigning each ASC with a unique set of channelization codes andsubchannels of a physical random access channel (PRACH), the subchannelsassociated with a single time slot of radio frames of the wirelessslotted communication system.
 6. The method of claim 5 wherein theunique set of channelization codes and subchannels of the PRACH define apartition of that PRACH.
 7. The method of claim 5 wherein the radioframes have system frame numbers (SFNs) and each subchannel isassociated with specified ones of the SFNs.
 8. The method of claim 5wherein the associating each subchannel is by a modulo counting of theSFNs.
 9. The method of claim 8 wherein a number of the subchannels is Nand N has a value of either 1, 2, 4 or
 8. 10. The method of claim 9wherein an i^(th) subchannel is associated with a SFN by i=SFN mod N. N.